Poker is a family of card games that share betting rules and usually (but not always) hand rankings. Poker games differ in how the cards are dealt, how hands may be formed, whether the high or low hand wins the pot in a showdown (in some games, the pot is split between the high and low hands), limits on bets and how many rounds of betting are allowed. In most modern poker games, the first round of betting begins with some form of forced bet. The action then proceeds to the left. Each player in turn must either match the maximum previous bet or fold, losing the amount bet so far and all further interest in the hand. A player who matches a bet may also raise, increasing the bet. The betting round ends when all players have either matched the last bet or folded. If all but one player fold on any round, the remaining player collects the pot and may choose to show or conceal their hand. If more than one player remains in contention after the final betting round, the hands are shown and the winning hand takes the pot.

With the exception of initial forced bets, money is only placed into the pot voluntarily by a player who, at least in theory, rationally believes the bet has positive expected value. Thus, while the outcome of any particular hand is determined mostly by chance, the long-run expectations of the players are determined by their actions chosen based on probability and psychology.

Beginning in 1970 a series of developments led to poker becoming far more popular than it was previously:

* Modern tournament play became popular in American casinos after the World Series of Poker began, in 1970. Notable champions from these early WSOP tournaments include Johnny Moss, Amarillo Slim, Bobby Baldwin, Doyle Brunson, and Puggy Pearson. * Later in the 1970s, the first serious strategy books appeared, notably Super/System by Doyle Brunson (ISBN 1-58042-081-8) and Caro's Book of Poker Tells by Mike Caro (ISBN 0-89746-100-2), followed later by The Theory of Poker by David Sklansky (ISBN 1-880685-00-0). * In the 1990s, poker and casino gambling spread across the United States, most notably to Atlantic City, New Jersey. * In 1998, Planet Poker dealt the first real money online poker game. * In 1999, Late Night Poker debuted on British television, introducing poker for the first time to many Europeans.